Guys, It is not a bug or anything, it is the fact that if you are using VPN or there's any firewall in the network you are connected to, the installer won't be able to connect to some apple download servers (on certain ports) to download necessary sources. I tried multiple times to update from 11.4 to 11.5 and then 11.6 to no avail in the office. Safe boot, restarts, App Store, etc. didn't help at all - it got stuck at "10 min remaining" for ages. I got 2 and 2 together when I read that some of people killed sophos (which filtered network traffic and probably ports) and I also knew my office network has VPN behind and NATs to allow access to remote infrastructure. So I went home and tried the same thing, no restart, no safe boot and - boom, no problem updating. Try performing the update in a NAT, VPN and firewall free network, and update will go through, I am writing from a big sur 11.6 at the moment. Good luck!